Why I-123 Is Used for Diagnostic Thyroid Imaging
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Question
This series of I-123 total body scans demonstrates normal biodistribution of radioiodine over time. Why is I-123 preferred over I-131 for DIAGNOSTIC thyroid imaging?
- AI-123 has a longer physical half-life than I-131, allowing imaging sessions to be spread over several days
- BI-123 emits energetic beta particles in addition to gamma rays, which improves gamma camera detection efficiency
- CI-123 distributes uniformly to all body tissues rather than concentrating selectively in the thyroid, reducing background noise
- DI-123 emits only gamma radiation with a 13.2-hour half-life, delivering a much lower radiation dose than I-131
Show the answer and explanation
D. I-123 emits only gamma radiation with a 13.2-hour half-life, delivering a much lower radiation dose than I-131
I-123 is preferred for diagnostic thyroid imaging because: (1) its half-life is 13.2 hours — long enough for imaging but short enough for rapid patient clearance; (2) it emits only gamma radiation (159 keV) with no beta particles, resulting in minimal absorbed dose to the thyroid. I-131 (half-life 8.02 days, emits both beta and gamma) delivers significant thyroid dose — acceptable for therapy but undesirable for diagnostics. I-131 is still used for post-ablation scanning in thyroid cancer, where its longer half-life aids in detecting distant metastases.
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